Modulation is the fundamental process in any communication system. It is a process to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A band-limited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The band-limited message is preserved, i.e., every frequency in that message is scaled by a constant value, and then transmitted by a transmitter. The three key parameters of a carrier wave are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal. The receiver then must select the correct transmitted signal from all the signals being transmitted by other transmitters in the area, and demodulate the signal using the same parameters the baseband signal was modulated with.
There are various shapes and forms of modulators and demodulators. For example conventional Amplitude Modulation uses a number of different techniques for modulating and demodulating the amplitude of the carrier in accordance with the information signal. These techniques have been described in detail in “Modern Analog and Digital Communication Systems” by B. P. Lathi. Similarly conventional Frequency/Phase Modulation uses a number of different methods described in a number of textbooks. In all these techniques, carrier (which is a high frequency sinusoidal signal) characteristics (either amplitude, frequency, phase or combination of these) are changed in accordance with the data (or information signal) by the modulator and changed back by the demodulator after transmission.
Communication systems that have emerged in recent years include mono-pulse and Ultra-Wide Band communication systems. The problem with these systems is that all mono-pulse or Ultra-Wide Band communications systems form Power Spectrum Densities that tend to span very wide swaths of the radio spectrum. For instance the FCC has conditionally allowed limited power use of UWB from 3.2 GHz to 10 GHz. These systems must make use of very wide sections of radio spectrum because the transmit power in any narrow section of the spectrum is very low. Generally any 4 KHz section of the affected spectrum will contain no more than −42 dbm of UWB spectral power. Correlating receivers are used to “gather” such very wide spectral power and concentrate it into detectable pulses. Interfering signals are problematic. Since the communication system is receiving energy over a very wide spectrum, any interfering signal in that spectrum must be tolerated and mitigated within the receiver. Many schemes exist to mitigate the interference. Some of these include selective blocking of certain sections of spectrum so as not to hear the interferer, OFDM schemes that send redundant copies of the information in the hope that at least one copy will get through interference, and other more exotic schemes that require sophisticated DSP algorithms to perform advanced filtering. In addition, UWB systems have somewhat of a “bad reputation” because they at least have the potential to cause interference. A heated discourse has gone on for years over the potential that UWB systems can cause interference to legacy spectrum users.
Tri-State Integer Cycle Modulation (TICM) and other Integer Cycle Modulation techniques were invented by Joe Bobier to help alleviate this massive and growing problem which has now become known by its commercial designation, xG Flash Signaling. Its signal characteristics are such that absolute minimal sideband energy is generated during modulation but that its power spectrum density is quite wide relative to the information rate applied. Also, a narrower section of the power spectrum output can be used to represent the same information. The technique of receiver synchronization disclosed herein is primarily applicable to these types of single cycle systems and Ultra Wide Band systems.
In a wireless network using TDMA for multiple access, each user is assigned a time slot for transmission and reception. The receiver in these systems has to acquire and maintain precise synchronization with the transmitter in order to accurately extract the payload data from the received data. The receiver needs to determine symbol timing, carrier frequency offset, and carrier phase offset. The synchronization subsystem in the receiver determines these factors for each received burst of data.
The synchronization subsystem has severe performance constraints in terms of processing time. This in turn places heavy demands on DSP performance and the digital acquisition and data transfer system itself. The amount of time this search takes directly relates to the time slot requirements and the throughput of the receiver.
A traditional TDMA receiver will start sampling data when in its intended timeslot. An entire timeslot's worth of data is presented to the DSP. Every burst of data contains a known sequence of bits that precede the payload data, called the preamble. The synchronization subsystem will start searching for the preamble starting from the very first sample it received. Various methods are used to determine the location of the preamble, such as correlation, MLSE, etc.
The time it takes to process and search through all the samples from the start of the timeslot to the start of the preamble can be called convergence time. The time taken by the digital acquisition system to transfer an entire timeslot's worth of data can be called data transfer time.
The disclosure of this application provides a method, called Symbol Sensed Synchronization (SSS), that significantly reduces the convergence time for synchronization. This method also significantly reduces the transfer time, and the memory required for data storage.